Solder preform and a process for its manufacture

ABSTRACT

A mixed mother alloy is prepared from a solder mixture comprising a pyrolyzable flux and high melting point metal particles, the mixed mother alloy is charged into a large amount of molten solder and stirred, and a billet is prepared. The billet can then be extruded, rolled, and punched to form a pellet or a washer, for example.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.12/073,690 filed on Mar. 7, 2008, now U.S. Pat. No. 7,793,820, which isa continuation-in-part of International Application Ser. No.PCT/JP2006/318248 having an international filing date of Sep. 14, 2006.

BACKGROUND OF THE INVENTION

This invention relates to a solder preform having high melting pointmetal particles uniformly dispersed therein and a process for itsmanufacture.

A typical soldering method for constituent parts such as printed circuitboards and electronic parts used in electronic equipment (referred tobelow as electronic parts or simply as parts) is the reflow solderingmethod.

The reflow soldering method is a method in which solder is placed onlyon necessary locations of a part, and then heating is performed by aheating apparatus such as a reflow furnace, an infrared irradiatingapparatus, or a laser irradiating apparatus to carry out soldering. Thereflow method not only has excellent productivity but can also carry outsoldering with excellent reliability in that solder is not adhered tounnecessary locations. Therefore, it is much used in soldering of modernelectronic parts which require high reliability.

Solders which are used in the reflow method include solder paste andsolder preforms. Solder paste is formed by mixing a viscous flux andsolder powder. It is applied by printing or dispensing to portions towhich electronic parts are soldered. Flux which is used in solder pastehas solids such as rosin and an activator dissolved in a solvent.Therefore, flux residue always adheres to soldered portions aftersoldering with a solder paste. If flux residue absorbs moisture in theatmosphere, it may produce corrosive products in soldered portions or adeterioration in insulation resistance. Therefore, parts which aresoldered with solder paste must generally undergo cleaning of fluxresidue when high reliability is demanded.

In order to solder electronic parts requiring high reliability, solderpreforms which enable soldering without employing flux are used. Asolder preform is solder which is previously formed (preformed) into ashape such as a pellet or a washer suited to the portion to be soldered.In a reflow method using a solder preform, the solder preform is placedon a portion to be soldered of an electronic part, and then the part isheated in a reducing atmosphere such as hydrogen gas to performsoldering. If an electronic part having a solder preform placed thereonis heated in a hydrogen atmosphere, the hydrogen acts to reduce andremove oxides adhering to the surface of the portion to be soldered ofthe part and the surface of the solder preform, thereby allowing themolten solder to wet the surface of the portion to be soldered.

A typical soldering technique which uses a solder preform is diebonding. Die bonding is joining of electronic parts such as a substrateand a semiconductor element with solder. Soldering is carried out byplacing a solder preform between the substrate and the semiconductorelement followed by heating in a reducing atmosphere.

If parts demanding high reliability are soldered with a solder preformwithout using flux, the problem of corrosion due to absorption ofmoisture by flux residue does not occur. Even so, corrosion of solderedportions sometimes becomes a problem. This type of corrosion is due tomoisture condensation. If the periphery of a soldered part is subjectedto a heat cycle of high temperatures and low temperatures, when thetemperature of the part decreases from a high temperature to a lowtemperature, moisture in the periphery of the part condenses, and waterdroplets adhere to the soldered portion. In a soldered portion, theionization tendency of the solder alloy is different from that of themetal of the portion to be soldered of the electronic part. As a result,the adhered water droplets dissolve electrolytes and form a local cell,and the solder or the metal of the part may corrode. In order to preventcorrosion due to moisture in parts demanding high reliability, resinmolding or potting in which the entire part is covered with a resin iscarried out.

When a solder preform and a semiconductor element are placed on asubstrate and heated so that the solder preform melts, the molten soldermay be forced out from the desired soldered portions of the parts due tothe weight of the semiconductor element, parts such as heat sinks, andjigs or the like, and the amount of solder present between the portionsto be soldered may end up becoming small. Joining by soldering canprovide a sufficient bonding strength to the extent that a suitableamount of solder is present between portions to be soldered, but ifsolder is forced out from between portions to be soldered by the weightof a semiconductor element as in die bonding and the amount of solderbecomes small, the bonding strength becomes weak.

In order to provide a suitable clearance between portions to be solderedand ensure that a suitable amount of solder is present between portionsto be soldered, a technique has been employed in which a plurality ofspherical particles of a high melting point metal having a melting pointhigher than solder such as Ni, Cu, Ag, Fe, Mo, and W (referred to belowsimply as metal particles) are sandwiched between portions to besoldered. For this purpose, solder preforms which already have metalparticles dispersed therein have been used, since it is extremelytroublesome and inefficient to separately place discrete metal particlesbetween portions to be soldered at the time of soldering.

Methods of manufacturing solder preforms having metal particlesdispersed therein include the cladding method and the melting method.

In the cladding method, a large number of metal particles are placedatop a single solder sheet, the sheet is passed between a pair ofrollers to embed the metal particles in the solder sheet, and the sheetis then subjected to punching with a press (see JP H03-281088 A1).Alternatively, metal particles are placed between two solder sheets toform a sandwich, which is then subjected to punching with a press (seeJP H06-285686 A1, for example).

In the melting method, metal particles are dispersed in molten solder,and the molten solder is then cast into a mold to form a billet. Thebillet is extruded to form a solder sheet, and the sheet undergoespunching with a press (see JP H06-31486 A1, for example). In the meltingmethod disclosed in JP H06-31486 A1, the surface of metal particles isfirst treated by electroplating or electroless plating. A mixture of themetal particles and flux is then charged into molten solder and stirred,and then the molten solder is cast into a mold to form a billet. Thebillet is then rolled to form a sheet, and the sheet is formed intosolder preforms of a predetermined shape with a press.

Because a solder preform which is obtained by the cladding method hasmetal particles mechanically embedded in a solder sheet or sandwichedbetween solder sheets, the metal particles have not been wet by moltensolder. Namely, the metal particles and the solder are not metallicallybonded to each other. Therefore, if such a solder preform is placedbetween portions to be soldered of electronic parts and the solderpreform is melted, a metallic bond is not formed where the metalparticles and the portions to be soldered are merely touching. Thisstate decreases the bonding area between the metal particles and solderand causes voids. As a result, not only is the bonding strengthinadequate, but the heat dissipation capacity decreases.

Heat dissipation capacity as used herein refers to, in the case ofsoldering of an electronic part such as a power transistor to a heatsink, for example, the ability to efficiently release heat, which isgenerated by the electronic part, through the heat sink. Heatdissipation capacity is greatly affected by thermal conductivity in thesoldered portions. By improving the heat dissipation capacity, adeterioration in performance of an electronic part due to a temperatureincrease of the part is prevented.

Particularly in the case of such a heat generating part, if there is notcomplete bonding between the electronic part and the heat sink, thebonding area becomes small and voids develop, thereby making heatconduction inadequate, and this produces thermal effects on electronicparts.

In solder preforms manufactured by the conventional melting method,since metal particles have been mixed with flux before they are chargedinto molten solder, it was expected that the metal particles and thesolder are metallically bonded to each other to obtain a sufficientbonding strength. However, the bonding strength after soldering wasinsufficient. When a solder preform obtained by the conventional meltingmethod is used for soldering by sandwiching it between parts and theinterior of the soldered portions is observed with an x-ray transmissionapparatus, voids which were not visible prior to soldering appear aftersoldering.

If voids develop in soldered portions, in the same manner as with solderpreforms manufactured by the cladding method in which metal particlesare not metallically bonded to solder, the bonding area becomes small,and not only does the bonding strength and the heat dissipation capacitydecrease, but voids expand due to the heat at the time of soldering andparts sometimes end up tilting.

With solder preforms manufactured by the conventional melting method, ithas been sometimes observed after soldering that flux oozes out to theperiphery of soldered portions. If flux oozes out to the periphery ofsoldered portions, the flux causes corrosion. In addition, when resinmolding or potting is carried out in order to protect soldered portionsagainst moisture, the flux is mixed into the resin and may interferewith curing of the resin.

In light of the above circumstances, there exists a need for a solderpreform containing metal particles which does not develop voids at thetime of solder bonding of parts and which does not experience a decreasein strength or a decrease in corrosion resistance and a process for itsmanufacture.

SUMMARY OF THE INVENTION

The present inventors focused on the melting method. As a result ofdiligent investigation of the cause of the occurrence of voids andoozing of flux to the periphery of soldered portions with a solderpreform obtained by the conventional melting method, they found thatthere is a problem with flux used when manufacturing solder preforms andwith a process for manufacturing solder preforms.

In general, it has been thought that flux completely vaporizes when itis exposed to a high temperature at the time of soldering. However,according to the findings of the present inventors, when flux used inordinary soldering is mixed with metal particles and charged into moltensolder, even if the flux is heated to a high temperature, it does notcompletely vaporize, and a minute amount of flux remains adhering to thesurface of the metal particles.

In the conventional melting method, a mixture of flux and metalparticles is directly charged into a large amount of molten solder andstirred, and the mixture is made into a billet. Therefore, a stirringoperation is carried out only one time, and it was found that fluxadhering to metal particles is not completely eliminated.

If even a minute amount of flux remains in a solder preform, even thoughthe flux itself does not become voids, when such a solder preform issandwiched between the portions to be soldered of parts and heated, theflux vaporizes and produces voids.

Namely, when flux is mixed with metal particles, solids in the flux suchas rosin, an activator, and a thixotropic agent are dissolved in asolvent. When the flux is charged into molten solder, a large portion ofthe solvent vaporizes, and the solids remain as flux. Flux remaining ina solder preform is a solid at room temperature, but when the remainingflux is heated at the time of soldering, it is liquified and thenvaporized. When flux undergoes a phase change from a liquid to a gas,its volume increases by several thousand times, and even a minute amountof flux forms large voids. Therefore, as described above, with a solderpreform in which a minute amount of flux remains, the bonding areadecreases, thereby decreasing the bonding strength and heat dissipationcapacity and causing parts to tilt. In addition, when a solder preformis melted, flux remaining in the solder preform oozes out from thesolder and adheres to the periphery of soldered portions and has aneffect on the curing of resins used for resin molding or potting.

Flux adhering to metal particles can be removed from the metal particlesby thoroughly carrying out stirring. However, if metal particles arecontacted with molten solder for a long period of time for the purposeof stirring, the metal particles are eroded by molten solder, as aresult of which the metal particles become small or completely melt intothe molten solder.

The present inventors found that if stirring is performed a plurality oftimes for a short period after metal particles are charged into moltensolder, flux adhering to the metal particles can be entirely removed andmetal particles are no longer eroded by molten solder. In addition, ifcomponents which are easily decomposed by the heat of molten solder areused to form a flux which is mixed with metal particles, the flux willno longer remain in a solder preform.

The present inventors also found that if metal particles are directlycharged into molten solder, a long time is required for uniformdispersion of metal particles, and erosion of metal particlesprogresses. Such problems can be solved by previously mixing metalparticles and a small amount of molten solder to prepare a mixed motheralloy and charging the mixed mother alloy into molten solder. The fluxremaining at the time of preparation of the mixed mother alloy can alsobe completely eliminated when subsequently charging the mixed motheralloy into molten solder.

The present invention was completed based on such findings. According toone aspect of the present invention, a process for manufacturing asolder preform comprises the following steps:

(1) mixing high melting point metal particles and a pyrolyzable liquidflux to form a mixture;

(2) preparing a mixed mother alloy by charging the mixture into moltensolder followed by stirring;

(3) preparing a billet by charging the mixed mother alloy into moltensolder followed by stirring and casting into a mold;

(4) forming the billet into a member suitable for punching; and

(5) punching the member to form a solder preform.

According to another aspect of the present invention, a solder preformhas high melting point metal particles uniformly dispersed in the solderpreform. Substantially no voids or flux are present in the solderpreform, and the surfaces of the high melting point particles aremetallically bonded to solder.

A mixed mother alloy which is employed in the present invention and amother alloy which is typically used when preparing a metal alloy havesimilar methods of use, but they have different make-ups. A mother alloyused in a typical alloy contains a high concentration of an alloyingmetal element which is added to molten metal and is completely dissolvedtherein, and the mother alloy is diluted such that a predeterminedcomposition is obtained. In contrast, a mixed mother alloy used in thepresent invention contains a large amount of high melting point metalparticles mixed and dispersed in solder in the form of unmelted metalparticles. When the actual amount of the metal particles in solder isobtained, the required amount of the mixed mother alloy is measured andcharged into molten solder.

There are no particular restrictions on the mixing ratio of the mixedmother alloy and molten solder as long as the amount of high meltingpoint metal particles which are supplied is sufficient to uniformlydisperse the high melting point metal particles in the solder preformwhich is finally obtained. At this time, the alloy composition of thematrix phase of the mixed mother alloy and that of the molten solder arepreferably the same, but it is also possible for them to have differentalloy compositions from each other. However, even when theircompositions are different, the compositions and mixing ratio arepreferably previously adjusted so that the solder alloy after meltinghas a predetermined composition.

In a process for manufacturing a solder preform according to the presentinvention, a mixture of flux and metal particles is charged into moltensolder and then stirred and rapidly cooled when preparing a mixed motheralloy, and the mixed mother alloy is then charged into molten solder andagain stirred and rapidly cooled when preparing a billet. Therefore, astirring operation is carried out two times, and flux adhering to metalparticles is thereby completely removed. In addition, flux used in amanufacturing process according to the present invention easilyundergoes pyrolysis (thermal decomposition). Therefore, when the mixtureis charged into molten solder or when the mixed mother alloy is chargedinto molten solder, the flux completely decomposes and does not remainin solder. Accordingly, when carrying out soldering of parts with asolder preform obtained by the manufacturing process according to thepresent invention, not only is there substantially no occurrence ofvoids, but there also is no oozing of flux to the periphery of solderedportions.

As a solder preform according to the present invention has metalparticles uniformly dispersed therein, when parts are soldered, aconstant clearance is obtained between the portions being soldered ofthe parts, and substantially no voids are present in the solder preform.Therefore, the inherent bonding strength of solder can be exhibited.

In addition, because a solder preform according to the present inventiondoes not have oozing of flux to the periphery of soldered portions, thecuring of resin is not impeded even if resin molding or potting iscarried out after soldering. As a result, soldered portions of excellentreliability are obtained.

A solder preform according to the present invention is not restricted toany particular shape or size. The most common shapes of solder preformsfor use with semiconductor devices are circular disks and rectangularpellets having a rectangular outline (which includes a square outline)as viewed in plan. The diameter of a disk-shaped preform is generally inthe range of 3-200 mm, and the dimensions of the sides of a rectangularpreform are typically 3-200 mm per side. The thickness of a solderpreform is typically 30-500 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows heating weight loss curves obtained by TG(thermogravimetry) of a pyrolyzable flux and a non-pyrolyzable flux.

FIG. 2 is a schematic view for explaining the state of dispersion ofhigh melting point metal particles in a pellet-shaped solder preformaccording to the present invention.

FIG. 3 is a schematic view for explaining the state of dispersion ofhigh melting point metal particles in a conventional pellet-shapedsolder preform.

FIG. 4 is a schematic cross-sectional view of a high melting point metalparticle having an intermetallic compound layer formed on its surface.

FIG. 5 is a schematic vertical cross-sectional view of a portion of asolder preform according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Flux used in a process for manufacturing a solder preform according tothe present invention must be flux which completely vaporizes orseparates by the heat of molten solder when it is mixed with metalparticles and charged into molten solder. Namely, the flux must be onein which the rosin, activator, and solvent which are componentsconstituting the flux completely vaporize and decompose or separate atthe melting temperature of solder.

Examples of a rosin which can be used as a flux component in the presentinvention are hydrogenated rosin, phenol modified rosin ester, andpolymerized rosin; examples of an activator are adipic acid, succinicacid, maleic acid, benzoic acid, and diethylaniline HBr; examples of asolvent are ethylene glycol monomethyl ether, ethylene glycol monoethylether, ethylene glycol monobutyl ether, diethylene glycol monomethylether, diethylene glycol monethyl ether, and 2-propanol.

The above-described rosins and activators of the flux decompose orseparate from the solder surface and do not remain in the solder at allat 285° C. which is the temperature at which the metal particles aredispersed in a Sn—Cu—Ni—P based lead-free solder (melting point ofapproximately 230° C.) comprising 99 mass percent of Sn and a remainderof Cu, Ni, and a minute amount of P. The above-described solvents have aboiling point of 230° C. or less, and they completely vaporize at themelting temperature of the above-described lead-free solder and does notremain in solder. One example of a preferred flux composition used inthe present invention is as follows. The flux of this example has 80%loss in weight at 285° C. on a heating weight loss curve obtained by TG.

60.0 mass percent of rosin (hydrogenated rosin),

10.0 mass percent of activator (diethylaniline HBr), and

30.0 mass percent of solvent (ethylene glycol monoethyl ether).

A pyrolyzable flux which is used in the present invention has a weightloss of at least 80% at 285° C. on a heating weight loss curve obtainedby TG as shown by the solid line in FIG. 1.

Lead-free solders having Sn as a main component which are much used atpresent have a liquid us temperature in the vicinity of 230° C. When amixture of flux and metal particles is charged into such a lead-freesolder to prepare a mixed mother alloy, the lead-free solder is heatedto a temperature of at least 285° C. Therefore, the flux in the mixturemust be one which vaporizes to give a weight loss of at least 80% at atemperature of 285° C. The fact that at least 80% of the flux vaporizesat a temperature of 285° C. means that the remaining 20% floats on topof the molten solder, and the probability of the flux remaining in thesolder becomes small. As shown by the dashed line in FIG. 1, a flux usedin usual soldering (a non-pyrolyzable flux) has a weight loss ofapproximately 65% at 285° C. In this state, a large amount of fluxremains in molten solder. The heating weight loss curve obtained by TGis obtained by measuring the change in weight of a sample with athermobalance while the sample is heated at a constant rate oftemperature increase. The actual weight loss on heating of flux can beinferred from this curve.

In the present invention, metal particles are not directly charged intomolten solder so as to achieve a predetermined proportion of a solderpreform. Instead, metal particles are first mixed in a higher proportionto prepare a mixed mother alloy, and the mixed mother alloy is chargedinto molten solder. If a predetermined amount of metal particles isdirectly charged into a large amount of molten solder and stirring isperformed as in the conventional melting method, a long time is requireduntil the metal particles are dispersed in the molten solder. Duringthis period, the metal particles are eroded by molten solder, wherebythe metal particles have a gradually decreasing particle diameter andend up being consumed. However, by preparing a mixed mother alloy with ahigher mixing proportion of metal particles as in the present invention,the amount of solder to be mixed with the metal particles becomessmaller. Therefore, all of the metal particles are easily dispersed inthe molten solder, and dispersing is carried out in a short length oftime. The length of time for which the metal particles contact moltensolder in the subsequent rapid cooling also becomes short, and erosionof the metal particles does not take place.

When the resulting mixed mother alloy is then charged into a largeamount of molten solder, since the metal particles are alreadymetallically bonded to solder in the mixed mother alloy, the mixedmother alloy is dispersed in a large amount of molten solder in a shortlength of time. In this case, the flux remaining in the mixed motheralloy is completely removed by the two occurrences of melting andstirring. Then, by pouring the molten solder having metal particlesuniformly dispersed therein into a mold for a billet and rapidly coolingthe mold, a billet having metal particles uniformly dispersed therein isobtained.

The concentration or density of high melting point metal particles inthe mixed mother alloy can be of a level such that high melting pointmetal particles are dispersed with a desired density when the mixedmother alloy is added to molten solder and melted. In general it is onthe order of 0.1-1 particle/mm². This concentration or density isdiluted by about 10 to 50 times with molten solder.

In a manufacturing process according to the present invention, apyrolyzable flux is used to form a mixture which is used when preparinga mixed mother alloy. For this reason, when the mixture is charged intomolten solder, almost all of the flux decomposes and vaporizes.Therefore, even if a minute amount of flux remains in the mixed motheralloy, when the mixed mother alloy is charged into molten solder in asubsequent step, the flux completely decomposes and none remains in thesolder. However, if it is desired to ensure that flux is completelyremoved from solder, at the time of preparing the mixed mother alloy ora billet, the molten solder can be placed in a vacuum apparatus and besubjected to vacuum treatment. In the present invention, vacuumtreatment is treatment which maintains molten solder at a reducedpressure. There is no particular limit on the level of vacuum at thistime, but in general, it is preferably approximately 10-100 Pa. Theduration of vacuum treatment is such that flux can be completelyremoved. For example, approximately 1-5 minutes are sufficient.

This vacuum treatment can be carried out at the time of preparing amixed mother alloy or at the time of preparing a billet or at bothtimes. If molten solder undergoes vacuum treatment in this manner, notonly flux present in the solder but also gases and impurities such asoxygen, nitrogen, intermixed oxides, and sulfides can be removed fromthe molten solder. As a result, solderability can be increased, and theoccurrence of voids can be substantially eliminated.

In the present invention, a mixed mother alloy is prepared by firstdispersing metal particles in a larger proportion than the proportion ofmetal particles to be dispersed in a solder preform to prepare a mixedmother alloy in which the metal particles are mixed in the state thatthey are wet by molten metal. A suitable mixing proportion of metalparticles in the mixed mother alloy is 2-30 mass percent.

When rectangular portions are soldered to each other, the number ofmetal particles which are dispersed to make it possible to maintain theportions to be soldered parallel to each other and leave a constantclearance between them is theoretically at least four such that there isone particle in each of four corners of the rectangle. However, if thefour metal particles are arranged in a straight line or if they collectin one location, portions to be soldered will not be parallel and aconstant clearance can not be maintained. Therefore, the metal particlesare preferably dispersed over the entire region of the portions to besoldered.

In this respect, when a square solder preform is divided into nine equalsmall squares, even if metal particles are not present in two of thenine small squares, if at least one metal particle is present in each ofthe remaining seven small squares, a part disposed atop the solderpreform does not tilt when the solder preform is melted.

Namely, when the square preform 1 shown in FIG. 2 is divided into nineequal small squares (a, b, c, d, e, f, g, h, i), even if metal particlesare not present in two of the small squares a and b of the nine smallsquares, it is sufficient for at least one metal particle 2 to bepresent in each of the remaining seven small squares c, d, e, f, g, h,and i. Even if metal particles are not present in the two small squaresa and b, for example, at the time of soldering, a part is maintainedparallel by the other small squares so that the part does not tilt. Inthe same manner, the part does not tilt even if metal particles are notpresent in two other small squares. However, as shown in FIG. 3, in thecase in which metal particles are not present in three of the smallsquares a, b, and c, a part ends up sloping when force is applied to oneside thereof.

Measurement of the number of metal particles can be carried out with anx-ray transmission apparatus.

In the case of disk-shaped preform having a circular periphery as viewedin plan, the nine squares which are used to evaluate the uniformity ofdistribution of particles in the preform define a square area measuring3 squares×3 squares, with the square area being circumscribed by thecircular periphery of the preform. In the case of a rectangular preformhaving a rectangular periphery which is not a square as viewed in plan,instead of dividing the preform into nine square areas, the entirepreform is divided into 9 rectangular areas of equal size. For example,in the case of a rectangular preform measuring 9×12 mm as viewed inplan, the preform is divided into 9 rectangular areas each measuring 3mm×4 mm, and the number of rectangular areas containing at least onemetal particle is determined.

High melting point metal particles dispersed in a solder preformaccording to the present invention must have a melting point higher thanthat of the solder preform, and the metal particles must be easily wetby molten solder. Examples of metal particles which can be used in thepresent invention are particles of Ni, Cu, Ag, Fe, Mo, and W, butpreferred metal particles for use in the present invention are Niparticles. Ni particles are not easily eroded by molten solder, and theyare inexpensive and readily commercially available as minute balls. Ofcourse, alloys of these metals may be used.

The clearance between soldered portions between which the metalparticles are sandwiched is not necessarily the same as the diameter ofthe metal particles, and it is normally slightly larger than thediameter of the metal particles. This is so that the metal particleswill not contact a part at the time of soldering and so that solder willbe present between the metal particles and the part. Therefore, theclearance is preferably larger than the diameter of the metal particlesby the amount of solder. However, when the weight of a part or a jig orthe like is large or a pressing force is applied to the part, almost allof the solder between the part and the metal particles disappears, andthe part contacts the metal particles. Even in such a state, the minimumclearance between a part and a portion being soldered must be at least20 μm. Namely, if the clearance between a part and a portion beingsoldered is less than 20 μm, the amount of solder becomes small, and theinherent bonding strength of solder can no longer be exhibited. For thisreason, the diameter of metal particles is at least 20 μm. Preferably itis from 40-300 μm.

In general, the thickness of a solder preform is roughly equal to thedesired clearance. Accordingly, the metal particles used in the solderpreform have a diameter close to the clearance. However, if thethickness of the solder preform and the diameter of the metal particlesare the same, at the time of forming a solder preform, the metalparticles end up being exposed on the surface of the solder preform, andsolder does not adhere to the exposed portions. As a result, a part isnot metallically bonded to the exposed portions of the metal particlesat the time of soldering. In the case of a thick solder preform, thediameter of metal particles is preferably made at most 90% of thethickness of the solder preform in order to leave molten metal whichcovers the upper and lower portions of the particles with solder in thethickness direction of the solder preform. FIG. 5 is a schematicvertical cross-sectional view of a portion of a solder preform 1according to the present invention showing a high melting point metalparticle 2 having solder 5 covering its upper and lower portions in thethickness direction of the solder preform 1.

A solder preform according to the present invention may comprise analloy of any composition, but a lead-free solder is particularlysuitable due to recent restrictions on the use of Pb. A lead-free solderhas Sn as a main component to which Ag, Cu, Sb, Bi, In, Zn, Ni, Cr, Mo,Fe, Ge, Ga, P and the like are suitably added. Sn tends to easily erodemetal particles, so when using Ni particles, Ni may be added to thelead-free solder in advance.

Namely, when using Ni particles as the high melting point metalparticles, if Ni is contained in the lead-free solder, it becomesdifficult for the Ni particles to be eroded when the molten lead-freesolder contacts the Ni particles. Examples of Ni-containing lead-freesolders include Sn—Cu—Ni—P based solders, Sn—Ag—Ni based solders,Sn—Cu—Ni based solders, Sn—Ag—Cu—Ni based solders, Sn—Zn—Ni basedsolders, Sn—Sb—Ni based solders, Sn—Bi—Ni based solders, and Sn—In—Nibased solders.

Molten solder which is prepared in this manner is cast into a mold andrapidly cooled to form a billet. The billet is next formed by extrudingusing an extruder into a member suitable for rolling, such as a strip.The extruded member is then subjected to rolling to form it into arolled member suitable for punching, such as a ribbon which is thinnerthan the strip. These operations themselves can be carried out inaccordance with the procedures employed in the already known meltingmethod, and there are no particular restrictions on them in the presentinvention.

The ribbon or other rolled member can be processed by punching with apress to form so-called solder preforms of various shapes such aspellets or washers.

A portion of a manufacturing process according to the present inventioncan be applied not only to the manufacture of a solder preform such as apellet or a washer but also to the manufacture of solder wire. Namely, amixture comprising a liquid flux and metal particles is mixed intomolten solder to prepare a mixed mother alloy, the mixed mother alloy ischarged into molten solder, and stirring is performed to manufacture abillet. By using the mixed mother alloy, high melting point metalparticles can be uniformly dispersed.

A billet which is obtained in this manner can then be subjected toextrusion and wire drawing to form wire-shaped solder such as solderwire or flux cored solder wire. Since the wire-shaped solder alsocontains metal particles, it can maintain the clearance of a solderedpart constant after soldering.

When the metal particles used to form a solder preform according to thepresent invention are in contact with solder (in either a solid orliquid state) at temperature high enough to promote diffusion ofelements forming the metal particles and the solder, a layer ofintermetallic compound(s) forms on the surface of the metal particles.For example, in the method according to the present invention, a layerof intermetallic compounds can form when the metal particles are mixedwith molten solder to form a mixed mother alloy and when the mixedmother alloy is charged into molten solder to prepare a molten materialfor casting. The thickness of the layer of intermetallic compound(s) isaffected by the temperature of the solder in contact with the metalparticles and the length of contact, with the thickness of the layerincreasing as either the temperature or the length of contact increases.The thickness of the intermetallic compound layer is also affected bythe flux which is mixed with metal particles when forming the mixedmother alloy, with the thickness of the intermetallic compound layerincreasing as the activity of the flux increases. FIG. 4 is a schematiccross-sectional view of a high melting point metal particle 3 having anintermetallic compound layer 4 formed over its entire surface.

The present inventors found that the layer of intermetallic compound(s)formed on the surface of a metal particle in a solder preform accordingto the present invention is preferably in the range of 0.05-10 μm andmore preferably in the range of 0.05-5 μm. If the thickness of theintermetallic compound layer is less than 0.05 μm, a large number ofvoids remain in the vicinity of the metal particles, resulting in adegradation in mechanical properties. On the other hand, if thethickness of the intermetallic compound layer exceeds 10 μm, the tensilestrength and the elongation of the resulting solder preform bothdecrease.

The thickness of the intermetallic compound layer can be adjusted bycontrolling the length of time for which the metal particles are incontact with solder at a temperature at which significant formation ofintermetallic compounds can take place. For example, one or more of thelength of time that the mixture of flux and metal particles is exposedto molten solder when forming a mixed mother alloy, the temperature ofthe molten solder used to form the mixed mother alloy, the cooling speedof the mixed mother alloy when it is cooled after being formed, thelength of time that the mixed mother alloy is stirred in molten solderto prepare a material for casting, the temperature of the molten solderto which the mixed mother alloy is added, the length of time that thematerial for casting sits prior to casting, and the cooling speed of thebillet formed by casting can be adjusted to vary the thickness of theintermetallic compound layer in the completed preform. Increasing any ofthese time periods or decreasing any of the cooling speeds will tend toincrease the thickness of the intermetallic compound layer, whiledecreasing any of these time periods or increasing any of the coolingspeeds will tend to have the opposite effect. Therefore, the timeperiods and cooling speeds can be adjusted empirically to obtain adesired layer thickness. It is also possible to increase the thicknessof the intermetallic compound layer after forming a billet by heatingthe billet or a member formed from the billet (such as a ribbon formedas an intermediate product or a preform itself) at a temperature belowthe melting point of the solder matrix but sufficient to promoteformation of intermetallic compounds.

There is no restriction on the composition of the intermetallic compoundlayer, and it may contain one or more intermetallic compounds. Thecomposition will depend upon the composition of the metal particles andthe solder forming the matrix of the preform. When the metal particlesare formed of Ni and the solder is a Sn-based solder, some examples ofintermetallic compounds which may be present in the intermetalliccompound layer are Ni₃Sn, Ni₃Sn₂, Ni₃Sn₄, and (Ni,Cu)₃Sn when the soldercontains Cu.

The thickness of the intermetallic compound layer formed on a metalparticle can be determined by observing a cross section of a preformcontaining a metal particle with an electron microscope and measuringthe thickness of the intermetallic compound layer on the metal particlein the field of view. The thickness of the intermetallic compound layermay vary around the periphery of a metal particle, so the thickness ofthe intermetallic compound layer is defined as the average of themaximum thickness and the minimum thickness of the layer around thecircumference of the metal particle.

In a solder preform manufactured by the method according to the presentinvention, the thickness of the intermetallic compound layer willfrequently vary from one metal particle to another. It is not necessaryfor all of the metal particles in a preform to have an intermetalliccompound layer with a thickness in the range of 0.05-10 μm, butpreferably as many of the metal particles as possible have anintermetallic compound layer with a thickness in this range, and morepreferably substantially all of the metal particles in a preform (suchas at least 90%) have an intermetallic compound layer with a thicknessfalling into this range.

In a solder preform manufactured by the above-described conventionalcladding method, it is impossible to form an intermetallic compoundlayer having a thickness of at least 0.05 μm since the metal particlesin the preform are not wet by molten solder, with the result that thebonding strength between the metal particles and the solder sheetsbetween which the particles are sandwiched is poor. In theabove-described conventional melting method in which high melting pointparticles are simply charged into molten solder and stirred, due to theconsiderable length of time for which metal particles are in contactwith molten solder when the molten solder is being stirred to uniformlydisperse the metal particles in the molten solder, the thickness of theintermetallic compound layer which forms on the metal particlesunavoidably becomes much greater than 10 μm.

In contrast, in a method of preparing a solder preform according to thepresent invention, metal particles are in contact with molten solder, soan intermetallic compound layer having a thickness of at least 0.05 μmcan be easily achieved. At the same time, the metal particles can berapidly dispersed in molten solder, so the length of time for which themetal particles are in contact with molten solder can be sufficientlyshort to limit the thickness of the intermetallic compound layer whichis formed to at most 10 μm. Next, examples of the present invention andcomparative examples will be explained.

Example 1

Solder preforms were prepared by the following steps (1)-(6).

(1) Ni particles with a diameter of 50 μm were washed with dilutehydrochloric acid to clean their surfaces, and 4.5 grams of the Niparticles which had their surfaces cleaned were mixed with 0.45 grams ofa pyrolyzable liquid flux having the following composition to obtain amixture.

The composition of the liquid flux used at this time was as follows. Itsweight loss at 285° C. was 80% on a heating weight loss curve obtainedby TG.

60.0 mass percent of rosin (hydrogenated rosin),

10.0 mass percent of activator (diethylaniline HBr), and

30.0 mass percent of solvent (ethylene glycol monoethyl ether).

(2) The mixture was charged into 125 g of a Sn—Cu—Ni—P based lead-freesolder (melting point of approximately 230° C.) comprising 99 masspercent of Sn and a remainder of Cu, Ni, and a minute amount of P whichhad been melted at approximately 285° C. in a cast iron ladle, and themolten solder was stirred for approximately 30 seconds with a metalspatula and then cast into a mold measuring 5×10×50 mm. The mold wasrapidly cooled with water to prepare a rod-shaped mixed mother alloy.This mixed mother alloy was washed with 2-propanol to remove fluxresidue which separated from the solder surface. The content of Niparticles in the mixed mother alloy was approximately 3.4 mass percent.

(3) Approximately 2.7 kg of a Sn—Cu—Ni—P based lead-free solder havingthe same composition as described above were melted by heating atapproximately 280° C. in a metal ladle, and 129.5 g of theabove-described mixed mother alloy were charged into the moltenlead-free solder. The molten solder was stirred for approximately 30seconds with a metal spatula. Then, the molten lead-free solder in whichNi particles were dispersed was cast into a cylindrical mold for abillet, and the mold was rapidly cooled with water to prepare a billet.

(4) The billet was extruded with an extruder to form a strip with athickness of 5 mm and a width of 20 mm.

(5) The strip was rolled in a rolling mill to a thickness of 0.2 mm anda width of 15 mm to form a ribbon.

(6) The ribbon was punched with a press to form pellet-shaped solderpreforms measuring 10×10 mm.

Example 2

Solder preforms were prepared by the following steps (1)-(6).

(1) Ni particles with a diameter of 50 μm were washed with dilutehydrochloric acid to clean their surfaces, and 4.5 g of the Ni particleswith cleaned surfaces were mixed with 0.45 g of a pyrolyzable liquidflux having the following composition to obtain a mixture.

The flux composition used in this example was as follows. Its weightloss at 285° C. was 80% on a heating weight loss curve obtained by TG.

60.0 mass percent of rosin (hydrogenated rosin),

10.0 mass percent of activator (diethylaniline HBr), and

30.0 mass percent of solvent (ethylene glycol monoethyl ether).

(2) The mixture was charged into 125 g of a Sn—Ag—Cu based lead-freesolder (melting point of approximately 220° C.) which had been melted at285° C. in a crucible, and after stirring was performed forapproximately 20 seconds with a metal spatula, the crucible was left ina vacuum apparatus for approximately 3 minutes to perform vacuumtreatment. Then, the crucible was removed from the vacuum apparatus andafter stirring was performed again for 5 seconds, the molten solder inwhich Ni particles were dispersed was then cast into a mold measuring5×10×50 mm. The mold was rapidly cooled with water to prepare arod-shaped mixed mother alloy. This mixed mother alloy was washed with2-propanol to remove flux residue which separated from the soldersurface. The content of Ni particles in this mixed mother alloy wasapproximately 3.4 mass percent.

(3) Approximately 2.7 kg of a Sn—Ag—Cu based lead-free solder was meltedby heating at 280° C. in a metal ladle, 129.5 g of the mixed motheralloy were charged into the molten lead-free solder, and stirring wasperformed with a metal spatula for approximately 30 seconds. Then, themolten lead-free solder in which Ni particles were dispersed was castinto a cylindrical mold for a billet, and the mold was rapidly cooledwith water to prepare a billet.

(4) The billet was extruded with an extruder to form a strip with athickness of 5 mm and a width of 20 mm.

(5) The strip was rolled with a rolling mill to a thickness of 0.2 mmand a width of 15 mm to form a ribbon.

(6) The ribbon was punched with a press to form pellet-shaped solderpreforms measuring 10×10 mm.

Comparative Example 1

Solder preforms were prepared by the following steps (1)-(5).

(1) Ni particles measuring 50 μm were washed with dilute hydrochloricacid to clean their surfaces, and 4.5 g of the Ni particles which hadtheir surfaces cleaned were mixed with 0.45 g of a liquid flux which isdifficult to pyrolyze and which had the following composition to preparea mixture.

The flux composition used in this example was as follows. Its weightloss at 285° C. was 65% on a heating weight loss curve obtained by TG.

60.0 mass percent of rosin (WW grade rosin),

10.0 mass percent of activator (diethylamine HBr), and

30.0 mass percent of solvent (diethylene glycol monobutyl ether).

(2) The mixture was charged into a large amount, i.e., approximately 2.7kg of a Sn—Cu—Ni—P based lead-free solder (melting point ofapproximately 230° C.) which had been melted at approximately 285° C. ina cast iron ladle and, and after stirring for approximately 30 secondswith a metal spatula, the molten lead-free solder was cast into acylindrical mold for a billet to prepare a billet.

(3) The billet was extruded with an extruder to form a strip with athickness of 5 mm and a width of 20 mm.

(4) The strip was rolled with a rolling mill to form a ribbon with athickness of 0.1 mm and a width of 15 mm.

(5) The ribbon was punched with a press to form pellet-shaped solderpreforms measuring 10×10 mm.

The solder preforms obtained by the above-described examples andcomparative example were observed with an x-ray transmission apparatus.The solder preforms obtained in the examples had a large number of Niparticles present in all of 9 equal small squares, and there were novoids.

A solder preform obtained in each of the examples was sandwiched betweena bare chip and a heat sink and heated at 280° C. in a hydrogenatmosphere to carry out soldering, and then the periphery of thesoldered portion was observed with a microscope. No oozing of flux wasobserved. There were also no voids when the soldered portion aftersoldering was observed with an x-ray transmission apparatus. The barechips and the heat sinks were parallel to each other, and the clearancewas 80 μm.

In contrast, a solder preform obtained in the comparative example didnot have Ni particles in 4 of the 9 equal small squares, and a largenumber of minute voids were present when the solder preform was observedwith an x-ray transmission apparatus.

When the solder preform was used for soldering under the same conditionsas for the examples, oozing of flux to the periphery of the solderedportions was observed. In addition, when the soldered portion wasobserved with an x-ray transmission apparatus, the presence of largevoids was observed.

Examples 3-5, Comparative Examples 2 and 3

In order to investigate the effects of the thickness of an intermetalliccompound layer formed on the surface of Ni particles in a solderpreform, a number of ribbons for use in manufacturing solder preformswere prepared. A tensile test specimen was cut from each ribbon andsubjected to a tensile test to determine its tensile strength and itselongation at failure in the tensile test. A cross section of eachribbon was observed with an electron microscope to determine thethickness of the intermetallic compound layer formed on the Niparticles. The voids rate in each specimen was measured using x-rayphotographs.

Examples 3-5 were prepared by the method according to the presentinvention comprising Steps (1)-(5) of Example 1. The thickness of theintermetallic compound layer was adjusted to 0.05 μm, 1 μm, or 10 μm byvarying the temperature of the molten solder bath and the mixing timewhen preparing a mixed mother alloy. Comparative Example 2 was preparedby a method similar to that used in Comparative Example 1. ComparativeExample 3 was prepared in a manner similar to that disclosed in JP2005-151338 in which Ni particles were disposed between two soldersheets, and the sheets were subjected to rolling to join the sheets toeach other.

The measured properties of the examples and comparative examples areshown in the following table. Each value in the table is the average fora plurality of specimens prepared by the same method.

Intermetallic Tensile compound layer Strength Elongation Voids ExampleNo. thickness (μm) (MPa) (%) (%) Example 3 0.05 20.3 35 5 Example 4 125.6 41.5 0.1 Example 5 10 22.5 38.1 0.1 Comparative 20 18.5 36 0.1Example 2 Comparative 0 18.4 34.6 7 Example 3

All of Examples 3-5, which had an intermetallic compound layer with athickness of 0.05-10 μm, were superior to Comparative Examples 2 and 3with respect to tensile strength. Examples 4 and 5 were also superior toComparative Examples 2 and 3 with respect to elongation, while Example 3had an elongation falling between the values for Comparative Examples 2and 3. The tensile strength, elongation, and voids rate wereparticularly good for Example 4, which had an intermetallic compoundlayer with a thickness of 1 μm.

Comparative Example 3, which was prepared by the cladding method, had ahigh voids rate of 7% because no metallic bonding took place between theNi particles and the solder sheets between which the Ni particles weresandwiched.

The results for Example 3 show that even a very thin intermetalliccompound layer with a thickness of 0.05 μm can provide a significantdecrease in voids rate and an increase in tensile strength. For Examples4 and 5 and Comparative Example 2 which had an intermetallic compoundlayer with a thickness of 1 μm and above, the voids rate was anextremely low value of 0.1%, meaning that these ribbons containedsubstantially no voids.

According to the present invention, substantially no voids are observedin solder joints of soldered portions, metal particles are uniformlydispersed, an excellent heat dissipation capacity is provided inheat-generating parts and the like, and a constant clearance isprovided.

What is claimed is:
 1. A solder preform for providing a clearancebetween portions to be soldered comprising high melting point metalparticles dispersed in solder in an unordered manner and metallicallybonded to the solder and having their upper and lower portions coveredby solder, the high melting point metal particles having a melting pointhigher than the solder and a particle diameter of at least 20 μm and atmost 90% of the thickness of the solder preform.
 2. A solder preform asclaimed in claim 1 wherein the high melting point metal particles are Niparticles.
 3. A solder preform as claimed in claim 1 wherein the solderpreform comprises a washer.
 4. A solder preform as claimed in claim 1wherein the solder preform comprises a pellet.
 5. A solder preform asclaimed in claim 1 wherein the solder preform can be divided into 9equal small squares with at least one of the high melting point metalparticles being present in at least 7 of the 9 squares.
 6. A solderpreform as claimed in claim 1 wherein no voids or flux is present in thesolder preform.
 7. A solder preform as claimed in claim 1 wherein thesolder preform measures 10×10 mm as viewed in plan and can be dividedinto 9 equal small squares each containing at least one of the highmelting point metal particles.
 8. A solder preform as claimed in claim 1wherein the solder preform measures 10×10 mm as viewed in plan and canbe divided into 9 equal small squares each containing a plurality of thehigh melting point metal particles.
 9. A solder preform as claimed inclaim 1 wherein every high melting point metal particle in the preformis spaced from top and bottom surfaces of the preform and is covered bysolder on its upper and lower portions.
 10. A solder preform having topand bottom surfaces and consisting of solder and a plurality of highmelting point metal particles dispersed in the solder in an unorderedmanner and metallically bonded to the solder and spaced from the top andbottom surfaces of the preform, the high melting point metal particleshaving a melting point higher than the solder and a particle diameter ofat least 20 μm and at most 90% of a thickness of the preform.
 11. Asolder preform as claimed in claim 1 wherein each of the high meltingpoint metal particles is covered with a solder layer over its entiresurface.
 12. A solder preform as claimed in claim 1 wherein the highmelting point metal particles are present in the solder as discreteparticles without being formed into chains.
 13. A solder preform asclaimed in claim 10 wherein each of the high melting point metalparticles is covered with a solder layer over its entire surface.
 14. Asolder preform as claimed in claim 10 wherein the high melting pointmetal particles are present in the solder as discrete particles withoutbeing formed into chains.